Dimerization of cyclopropenes to bifurans using tandem metal relay catalysis

Article abstract of DOI:10.1039/c3cc44762f

An efficient synthesis of bifurans via dimerization of cyclopropenes has been successfully developed using a copper-promoted cycloisomerization and palladium-catalyzed dimerization cascade. These novel bifuran structures possess interesting optoelectronic properties.

Full text of DOI:10.1039/c3cc44762f

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Dimerization of cyclopropenes to bifurans using
tandem metal relay catalysis†
Cite this: Chem. Commun., 2013,
49, 9167
Chuanling Song, Di Sun, Xianglong Peng, Jing Bai, Rongyi Zhang, Shengzhen Hou,
Jianwu Wang and Zhenghu Xu*
Received 25th June 2013,
Accepted 6th August 2013
DOI: 10.1039/c3cc44762f
www.rsc.org/chemcomm
An efficient synthesis of bifurans via dimerization of cyclopropenes has
been successfully developed using a copper-promoted cycloisomeriza-
tion and palladium-catalyzed dimerization cascade. These novel bifuran
structures possess interesting optoelectronic properties.
Fig. 1 Oligothiophene and oligofuran-based materials.
Organic molecules bearing a heteroaryl structure motif have
attracted great attention recently and are making significant
contributions in different fields such as organic light emitting
diodes (OLEDs),1 organic field effect transistors (OFETs),2 and
solar cells.3 Oligothiophene derivatives have been mostly high-
lighted in the past twenty years because of their superior electronic
properties.4 In 2010, Bendikov’s group first reported that the
oligofurans showed increased fluorescence, better packing and
greater solubility than the corresponding oligothiophenes (Fig. 1).5
Very recently, the same group investigated two basic units (bifuran
and bithiophene) by comparing two structural isomers: TFFT and
FTTF and found that the bifuran moiety is responsible for the
observed better properties.5c This work demonstrates that smaller
building blocks like bifurans have similar properties to longer
oligofurans. In principle, incorporation of the bifuran units into
organic materials should be considered a rational design of
structurally-complex conjugated organic electronic materials.
Therefore, efficient synthesis of these bifuran units is of great
importance. State of the art synthesis of these compounds relies on
the transition-metal catalyzed cross coupling reactions (Suzuki,
Negishi, Stille, etc.).5 In these traditional coupling reactions, both
heteroaryl organometallics and heteroaryl halides need to be pre-
pared in advance. Fagnou and others reported more efficient direct
arylation reactions by using aryl halides with heteroarenes.6 In this
approach, only one coupling partner must be preactivated
(Scheme 1). Very recently, the direct oxidative C–H/C–H coupling
of two heteroarenes has been reported as the most ideal synthetic
methodology.7 However, these reactions normally require very harsh
conditions to achieve the twofold C–H activation. Herein, we report
the synthesis of bifuran products through cycloisomerization/
dimerization of cyclopropenes via Tandem Metal Relay Catalysis
(TMRC).8 These novel multifunctionalized bifuran structures possess
interesting optoelectronic properties and have potential to find
applications in organic electronic materials.
Recently, we have developed an efficient copper–palladium relay
catalysis (TMRC) for the synthesis of tetra-substituted alkene
functionalized furans from cyclopropenes under very mild reaction
conditions (Scheme 1).8 Cyclopropene9 underwent a copper cata-
lyzed ring-opening/cycloisomerization reaction10 and subsequent
transmetallation to palladium, generating the key furan palladium
intermediate M. This intermediate underwent an oxidative Heck
reaction with alkenes generating the tetrasubstituted furans. Using
this strategy, the key carbon–palladium bond was formed through a
transmetallation strategy under very mild conditions. We then
extended this methodology to the oxidative carbonylation of cyclo-
propene for the synthesis of tetra-substituted furan carboxylate 3a.8b
Key Lab for Colloid and Interface Chemistry of Education Ministry,
School of chemistry and Chemical Engineering, Shandong University, Jinan 250100,
People’s Republic of China. E-mail: xuzh@sdu.edu.cn
† Electronic supplementary information (ESI) available: Detailed experimental
procedures and analytical data. CCDC 930435 for compound 2a. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3cc44762f
Scheme 1 Strategies for synthesis of biheteroaryl compounds.
Chem. Commun., 2013, 49, 9167--9169 9167
c
This journal is The Royal Society of Chemistry 2013

View Article Online
Communication
ChemComm
Scheme 2 Copper–palladium cooperative catalyzed oxidative dimerization of
cyclopropenes to bifurans.
To our great surprise, the bifuran product was obtained in 65%
yield at 80 1C (Scheme 2). The structure was unambiguously
characterized by NMR, mass analysis and also single X-ray
crystallography.
Examining this reaction in detail, we discovered that CO was
not needed. However, both copper and palladium were essential
for this transformation. Under optimized reaction conditions
(for details, see ESI†), Pd(OAc)2 (5 mol%) and Cu(OAc)2 (2 equiv.)
at 80 1C, bifuran 2a was formed in 86% yield in 3 h.
Several alkyl or aryl substituted cyclopropenes were subjected to
this reaction and the results are summarized in Table 1. Alkyl
substituted cyclopropene dicarboxylates reacted smoothly to generate
the corresponding bifurans in 65–89% yield within 3 h (2a–2e).
Cyclopropenes bearing other electron-withdrawing groups such as
CN and SO2Ph were all suitable substrates and generated the
corresponding functionalized bifurans in 63–72% yield (2g and
2h). In addition, cyclopropene derived from ethyl acetoacetate
worked well and furnished 2-alkyl substituted bifuran 2f in 47%
yield. Aryl substituted cyclopropenes were more reactive and the
Fig. 2 (a) Absorption and fluorescence spectra in DCM. (b) Solution lumines-
cence (2m) (upon irradiation at 365 nm).
bifuran products were obtained in moderate yield even at room
temperature (2g–2o). This methodology is superior to the existing
ones because normal direct C–H/C–H coupling reactions need very
high temperature.7 Remarkably, the halogen groups such as F and Br
on the phenyl ring were all tolerated under these mild conditions
(2m, 2n). Having halogens on the bifurans makes it possible to
couple with other functional groups or chromophores to give larger
Pi-extended conjugated functionalized molecules.
The UV/Vis absorption and emission spectra of the bifurans 2a,
2g, 2k and 2m in solution were characterized and are shown in
Fig. 2. Both the absorption and emission bands of the aromatic
substituted bifurans 2k and 2m are red shifted compared with
n-butyl group substituted bifurans 2a and 2g. In the fluorescence
spectra, 2k and 2m exhibited blue emission at 445 nm, while 2a and
2g exhibited emission at 405 nm. In addition, the fluorescence
efficiency of 2k is comparable to the long oligofurans (Ff = 0.97 in
DCM). n-Butyl substituted bifurans 2a and 2g showed blue emission
both at high concentration and in the solid state. By contrast, an
aggregation-caused quenching (ACQ) effect was observed for com-
pounds 2k and 2m as they only luminesce in highly dilute solutions
(see ESI†). These functionalized materials showed great promise as
blue emissive materials used as OLEDs.
Table 1 Substrate scope of cyclopropene dimerization to bifuransa
To gain some insight into the mechanism of the reaction, several
control experiments were conducted (Scheme 3). We observed that
even when the reaction was conducted at 60 1C, activity was still
good. The cyclopropene was consumed in 1 h giving 2a and some
trisubstituted furan 4a. When Cu(OAc)2ÁH2O was used instead of
anhydrous Cu(OAc)2, more furan 4a was obtained (92/8 vs. 77/23).
When the reaction was carried out at the standard 80 1C, no
isomerized furan 4a was observed. If furan 4a was subjected to
these standard conditions, bifuran 2a could be produced through
a
Reaction conditions: cyclopropene 1 (0.2 mmol), Pd(OAc)2 (5 mol%),
Cu(OAc)2 (0.4 mmol), EtOH (0.2 mL), CH3CN (1 mL), DMSO (1.4 mmol),
80 1C, 3 h. At room temperature, 12 h.
b
Scheme 3 Control experiments.
c
9168 Chem. Commun., 2013, 49, 9167--9169
This journal is The Royal Society of Chemistry 2013

View Article Online
ChemComm
Communication
P. Buerle, Chem. Rev., 2009, 109, 1141; (c) M. Teiber and T. J. J.
Mu¨ller, Chem. Commun., 2012, 48, 2080.
5 (a) O. Gidron, Y. Diskin-Posner and M. Bendikov, J. Am. Chem. Soc.,
2010, 132, 2148; (b) O. Gidron, A. Dadvand, Y. Sheynin, M. Bendikov
and D. F. Perepichka, Chem. Commun., 2011, 47, 1976; (c) O. Gidron,
N. Varsano, L. J. W. Shimon, G. Leitus and M. Bendikov, Chem.
Commun., 2013, 49, 6256; (d) X. Cui, X. Xu, L. Wojtas, M. M. Kim and
X. P. Zhang, J. Am. Chem. Soc., 2012, 134, 19981.
6 For reviews, see: (a) L.-C. Campeau and K. Fagnou, Chem. Commun.,
2006, 1253; (b) D. Alberico, M. E. Scott and M. Lautens, Chem. Rev.,
2007, 107, 174; (c) I. V. Seregin and V. Gevorgyan, Chem. Soc. Rev.,
2007, 36, 1173For selected examples, see: (d) L.-C. Campeau,
D. R. Stuart, J.-P. Leclerc, M. Bertrand-Laperle, E. Villemure,
H.-Y. Sun, S. Lasserre, N. Guimond, M. Lecavallier and K. Fagnou,
J. Am. Chem. Soc., 2009, 131, 3291; (e) D. J. Schipper and K. Fagnou,
Chem. Mater., 2011, 23, 1594; ( f ) H. Hachiya, K. Hirano, T. Satoh
and M. Miura, Angew. Chem., Int. Ed., 2010, 49, 2202; (g) D. Zhao,
W. Wang, F. Yang, J. Lan, L. Yang, G. Gao and J. You, Angew. Chem.,
Int. Ed., 2009, 48, 3296; (h) R. J. Phipps, N. P. Grimster and
M. J. Gaunt, J. Am. Chem. Soc., 2008, 130, 8172; (i) H.-Q. Do and
O. Daugulis, J. Am. Chem. Soc., 2007, 129, 12404; ( j) H.-Q. Do,
R. M. K. Khan and O. Daugulis, J. Am. Chem. Soc., 2008, 130, 15185;
(k) M. Ye, G. Gao, A. J. F. Edmunds, P. A. Worthington, J. A. Morris
and J.-Q. Yu, J. Am. Chem. Soc., 2011, 133, 19090; (l) M. Ye,
A. J. F. Edmunds, J. A. Morris, D. Sale, Y. Zhang and J.-Q. Yu, Chem.
Sci., 2013, 4, 2374, DOI: 10.1039/c3sc50184a.
7 For a recent review, see: (a) D. Zhao, J. You and C. Hu, Chem.–Eur. J.,
2011, 17, 5466For recent examples, see: (b) K. Masui, H. Ikegami and
A. Mori, J. Am. Chem. Soc., 2004, 126, 5074; (c) M. Takahashi,
K. Masui, H. Sekiguchi, N. Kobayashi, A. Mori, M. Funahashi and
N. Tamaoki, J. Am. Chem. Soc., 2006, 128, 10930; (d) J. Xia, X. Wang
and S.-L. You, J. Org. Chem., 2009, 74, 456; (e) H.-Q. Do and
O. Daugulis, J. Am. Chem. Soc., 2009, 131, 17052; ( f ) Z. Liang,
J. Zhao and Y. Zhang, J. Org. Chem., 2010, 75, 170; (g) P. Xi,
F. Yang, S. Qin, D. Zhao, J. Lan, G. Gao, C. Hu and J. You, J. Am.
Chem. Soc., 2010, 132, 1822; (h) Z. Wang, K. Li, D. Zhao, J. Lan and
J. You, Angew. Chem., Int. Ed., 2011, 50, 5365; (i) X. Qin, H. Liu,
D. Qin, Q. Wu, J. You, D. Zhao, Q. Guo, X. Huang and J. Lan, Chem.
Sci., 2013, 4, 1964.
Scheme 4 Proposed reaction mechanism.
palladium catalyzed C–H activation, but the reaction was very slow
and bifuran 2a was obtained in only 17% yield in 1 h, and most of
the starting 4a was recovered. These data suggest that there was an
equilibrium between the furan palladium intermediate M and furan
4a in the presence of water. In this reaction, the copper–palladium
transmetallation is still the major pathway, even though a cyclo-
isomerization to 4a/palladium catalyzed C–H activation/
dimerization pathway couldn’t be ruled out (Scheme 4).
A Tandem Metal Relay Catalysis (TMRC) mechanism is
proposed in Scheme 4. Cyclopropene reacts with Cu(OAc)2
generating the copper carbene intermediate B, followed by
intramolecular cyclization and elimination of HOAc leading to the
vinyl-copper intermediate D. Subsequent transmetallation generates
the key furan palladium intermediate M. The intermediate under-
goes disproportionation to afford Pd(OAc)2 and bifuran palladium
M1. This then undergoes reductive elimination to generate bifuran
2 and Pd(0), which was oxidized to Pd(II) by Cu(OAc)2.
In summary, we have developed a convenient and efficient
synthetic methodology toward multifunctionalized bifuran
products from cyclopropenes. The mild reaction conditions and
high efficiency are the most important features of this formal C–H/
C–H coupling transformation. The Tandem Metal Relay Catalysis
(TMRC) mechanism was proposed. The bifuran compounds hold
promise for applications as optoelectronic materials. Further work
seeking to synthesize and extend bifuran-based Pi-conjugated
molecules using this methodolgy and to find applications as
organic electronic materials is underway.
8 (a) C. Song, L. Ju, M. Wang, P. Liu, Y. Zhang, J. Wang and Z. Xu,
Chem.–Eur. J., 2013, 19, 3584; (b) C. Song, S. Dong, L. Feng, X. Peng,
M. Wang, J. Wang and Z. Xu, Org. Biomol. Chem., 2013, DOI: 10.1039/
c3ob41155a.
9 For recent reviews on cyclopropenes, see (a) M. Rubin, M. Rubina
and V. Gevorgyan, Chem. Rev., 2007, 107, 3117; (b) Z.-B. Zhu, Y. Wei
and M. Shi, Chem. Soc. Rev., 2011, 40, 5534; (c) F. Miege, C. Meyer
and J. Cossy, Beilstein J. Org. Chem., 2011, 7, 717; (d) M. Rubin,
M. Rubina and V. Gevorgyan, Synthesis, 2006, 1221; (e) I. Marek,
S. Simaan and A. Masarwa, Angew. Chem., Int. Ed., 2007, 46, 7364;
( f ) J. M. Fox and N. Yan, Curr. Org. Chem., 2005, 9, 719; For recent
advances, see: (g) Y. Liu and S. Ma, Chem. Sci., 2011, 2, 811;
(h) L. H. Phun, J. Aponte-Guzman and S. France, Angew. Chem.,
Int. Ed., 2012, 51, 3198; (i) Y. Liu and S. Ma, Org. Lett., 2012, 14, 720;
( j) F. Miege, C. Meyer and J. Cossy, Angew. Chem., Int. Ed., 2011,
50, 5932; (k) K. Kramer, P. Leong and M. Lautens, Org. Lett., 2011,
13, 819; (l) F. Liu, X. Bugaut, M. Schedler, R. Frohlich and F. Glorius,
Angew. Chem., Int. Ed., 2011, 50, 12626; (m) X. Bugaut, F. Liu and
F. Glorius, J. Am. Chem. Soc., 2011, 133, 8130; (n) Z. Zhu, K. Chen,
Y. Wei and M. Shi, Organometallics, 2011, 30, 627; (o) M. S. Hadfield
and A. L. Lee, Chem. Commun., 2011, 47, 1333; (p) D. H. T. Phan,
K. G. M. Kou and V. M. Dong, J. Am. Chem. Soc., 2010, 132, 16354;
(q) C. Li, H. Zhang, J. Feng, Y. Zhang and J. Wang, Org. Lett., 2010,
12, 3082; (r) V. Tarwade, X. Liu, N. Yan and J. M. Fox, J. Am. Chem.
Soc., 2009, 131, 5382; (s) S. Chuprakov, M. Rubin and V. Gevorgyan,
J. Am. Chem. Soc., 2005, 127, 3714; (t) Z. Zhu and M. Shi, J. Org.
Chem., 2009, 74, 2481; (u) L.-X. Shao, Y. Zhang, M. Qi and M. Shi,
Org. Lett., 2007, 9, 117.
We are grateful for financial support from the Natural
Science Foundation of China and Shandong Province (Grant
No. 21102085, BS2012YY006) and Ministry of Education of
China (No. 20110131120049).
Notes and references
1 (a) I. F. Perepichka, D. F. Perepichka, H. Meng and F. Wudl, Adv.
Mater., 2005, 17, 2281; (b) S. Reineke, F. Lindner, G. Schwartz,
N. Seidler, K. Walzer, B. Lussem and K. Leo, Nature, 2009, 459, 234.
2 (a) Organic Field Effect Transistors, ed. Z. Bao and J. Locklin, CRC 10 For copper promoted cyclopropene isomerization into furan or
Press, 2007; (b) W. Wu, Y. Liu and D. Zhu, Chem. Soc. Rev., 2010,
39, 1489.
3 (a) R. F. Service, Science, 2011, 332, 293; (b) Y.-J. Cheng, S.-H. Yang
and C.-S. Hsu, Chem. Rev., 2009, 109, 5868.
4 (a) Handbook of Thiophene-Based Materials, ed. I. F. Perepichka and
D. F. Perepichka, Wiley-VCH, 2009; (b) A. Mishra, C.-Q. Ma and
other heterocycles, see: (a) J. Chen and S. Ma, Chem.–Asian. J.,
2010, 5, 2415; (b) S. Ma and J. Zhang, J. Am. Chem. Soc., 2003,
125, 12386; (c) S. Chuprakov and V. Gevorgyan, Org. Lett., 2007,
9, 4463; (d) M. Rubin and P. Ryabchuk, Chem. Heterocycl. Compd.,
2012, 48, 126; (e) A. V. Gulevich, A. S. Dudnik, N. Chernyak and
V. Gevorgyan, Chem. Rev., 2013, 113, 3084.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 9167--9169 9169